The present invention relates generally to the field of steam generators and in particular to a new and useful circulation system for sliding pressure steam generators.
The design of once-through boilers has been around since 1926. The design of the once-through boiler was developed by Siemens on the basis of ideas that were proposed by Mark Benson. The Benson boiler introduced the concept of sliding pressure operation for a supercritical steam pressure design (e.g., to accommodate supercritical steam pressure). In the Benson design, the boiler feed pump provides the entire driving head to force the water through the economizer, evaporator, and superheater. Water is continuously evaporated to dryness and then superheated without any steam-water separation. The circulation method is applicable to all operating pressures both supercritical and subcritical. Typically, most applications of the Benson design use spiral furnace circuitry for the evaporator due to the fact that a vertical tube evaporator design is more sensitive to upsets and nonuniform tube-to-tube heating. For start-up and low load operation, special by-pass systems are needed.
In one example shown in FIG. 1, a boiler feed pump 908 for the system 910 provides the entire driving head to force the water through an economizer 911, an evaporator 912, and a superheater 914 which can be used in conjunction with a separator 913. Water is continuously evaporated to dryness and then superheated without any steam-water separation. This circulation method is applicable to all operating pressures, i.e. supercritical (greater than 3208 psia) and subcritical (less than 3208 psia). Typically, the system 910 uses spiral furnace circuitry for the evaporator 912 because a vertical tube design is more sensitive to upsets and nonuniform tube-to-tube heating. For start-up and low load operation, special by-pass systems are still needed.
In overcoming start-up and low load operation, many boiler manufacturers have developed once-through boiler designs with superimposed recirculation systems 910a and 910b illustrated in FIGS. 2 and 3. These recirculation systems permit partial recirculation of fluid to the furnace walls in order to increase the fluid velocity in the evaporator tubes by incorporating circulation pumps 915 and orifices 916. The design in many applications allows the furnace 912 to remain at constant pressure, typically supercritical pressure, and utilizes a separator or flash tank 913 for reducing the superheater pressure to subcritical pressures at start-up and low loads. These types of once-through boiler systems 910a and 910b typically utilize a vertical furnace tube evaporator design.
Examples of these types of units are B&W's Universal Pressure (UP) boiler, CE's Combined Circulation boiler and Foster Wheeler's Multipass boiler. These boilers permit partial recirculation of fluid to the furnace walls to increase the fluid velocity in the evaporator tubes. The design in many applications allows for the furnace to remain at constant pressure typically supercritical pressures and utilizes a separator or flash tank for reducing the superheater pressure to subcritical pressures at start-up and low loads. This type of once-through boiler typically utilizes a vertical furnace tube evaporator design.
Once-through boiler designs that utilize both spiral and vertical tube furnace evaporators have been sold by many boiler manufacturers, and developed for either supercritical or subcritical steam pressures. Vertical tube once-through boilers for sliding pressure applications are becoming more accepted in the industry. The sliding pressure operation of the vertical tube once-through boiler is restricted to specific minimum load due to the flow requirements of the evaporator. The spiral tube furnace does not have this restriction. The spiral furnace permits greater freedom in matching the tube diameter and mass velocity of the furnace to ensure tube cooling and flow stability in the parallel furnace evaporator tubes. It also allows each tube of the furnace to run through the various heat zones in the combustion chamber, so that difference in total heat input between tubes will be kept to a minimum.
The development of a vertical tube sliding pressure once-through boiler is needed due to the higher cost of the spiral furnace design when compared to a vertical furnace design. The construction of a forced circulation once-through boiler requires the use of a very large number of parallel tubes, welded together to form membrane panels. A fundamental requirement for membrane wall integrity is uniform fluid and metal temperature in all tubes at each furnace level. Until now, the major problem with the vertical tube design was due to the large heating difference between individual tubes in the furnace. In vertical tube furnaces the heating difference between tubes is approximately 2.5 times as great as in spiral furnace design. Average mass velocities of 1,500,000 to 2,000,000 lb/hr-ft**2 are typical velocities used in current once-through boiler designs. These mass velocities when subjected to typical peripheral furnace heat absorption variations (which can vary +/−35% or more from average) result in a velocity variation that decreases in magnitude with increasing heat input. This trend is called the once-through characteristic of a boiler tube. In the once-through mode, the velocity change due to an increase in heat is negative as shown in FIG. 4. In the event of excessive heat input to a single tube, a reduction in mass velocity occurs, causing an additional increase in the outlet temperature of the tube.
U.S. Pat. No. 5,390,631 teaches the use of Multi-Lead Ribbed (MLR) and Single-Lead Ribbed (SLR) tubing for the design of a vertical tube and spiral tube furnace sliding pressure once-through boiler. The location of each type of tubing in the furnace is determined based upon the heat transfer and flow characteristics for all loads at which the unit is expected to operate. This basically covers the range of loads from minimum load of approximately 25 to 30% of Maximum Continuous Rating (MCR) to MCR load. The novel furnace design consists of vertical smooth bore tubes in the low heat flux regions of the furnace and of a combination of vertical MLR and SLR tubes in the high heat flux areas, where necessary to avoid Departure from Nucleate Boiling (DNB) and/or Critical Heat Flux (CHF) and to meet tube metal temperature limitations. The length and the location of the MLR/SLR combination is adjusted for each panel to achieve optimum natural circulation characteristics. Since SLR tubes have higher flow resistance than MLR tubes or smooth tubes, their use must be minimized to locations where absolutely needed. Higher flow resistance has a tendency to reduce the desired natural circulation effect. But proper location and the correct proportion of the SLR and the MLR tubes around the furnace periphery will minimize the fluid and metal temperature difference between all membrane wall tubes at any elevation to stay below the allowable limit of 100 degrees F. at all loads. With natural circulation characteristics, the tubes in the furnace evaporator would have similar outlet temperatures in spite of the different heating characteristics of the vertical tube design. The actual design of the locations of each tubing type will be a function of the geometric size of the furnace, the kind and type of fuel, the load change requirements of the unit and the pressure and temperature requirements of the unit. The application of this concept could be distinct for each panel in the furnace. The location of the SLR tubing in one panel could be in a different elevation, either higher or lower, than the panel adjacent to it.
U.S. Pat. No. 5,713,311 teaches a hybrid steam generating system. This system utilizes a typical furnace with a circulation system that can be used as a natural circulation system with recirculation during low load operation from 0 to ˜25% load, a hybrid natural/once through circulation system from ˜25% to 50% load, and once through circulation system from ˜50% load to 100% load. The hybrid system allows for the unit to operate with natural circulation characteristics at low load and once through characteristics at higher loads. The system combines the operating principles of both the natural circulation drum boiler and once-through system.
U.S. Pat. No. 4,290,389 teaches a concept that is very similar to the concept given in FIGS. 2 and 3 of the prior art. The concept uses circulation pumps and orificing to achieve a sliding pressure once through boiler. The pumps are used for operation at lower loads and to satisfy the high pressure drop of the furnace orifices. The orificing is required for high pressure and high load operation.
Thermal-hydraulic problems are associated with the operational and design issues of once through boilers at reduced loads. These design issues are partially caused by the large variation in the furnace heat distribution and often require the use of orifices and/or circulation pumps to distribute the flow to the furnace circuits to correct the circulation problem. In many of the prior art designs, obtaining the necessary flow at low loads to adequately cool the furnace tubes required a much higher flow velocity than necessary at full load. Spiral tube furnace designs have also been used to minimize these effects of the heat absorption and load by properly selecting the furnace tube size and spiral angle to obtain an adequate furnace design. In both of the spiral and vertical tube boiler cases, high fluid velocities at full load were required to successfully design the unit for a typical load range from 35% to full load. The pressure drop associated with a furnace design with high velocities results in less efficient boiler design.
Other circulation design concerns with once through boilers occur at lower loads due to dynamic flow stability problems. Since the flow velocity is much lower at low loads, flow instability may occur which is undesirable for safe and reliable boiler operation. Correction of this problem has resulted in the orificing of each individual tube in the furnace. The orificing increases the pressure drop of the once through boiler design resulting in a less efficient boiler.
A recent design concept that has been developed by Siemens for a vertical tube furnace design requires the use of a special type of ribbed tube (optimized multi-ribbed tube) which will allow a lower mass velocity for the furnace design. However, the cost of this new ribbed tube is much greater than the normal type of ribbed tube. A design is needed in which the furnace tubing will only require standard MLR tubing.
A solution is needed for the issues with the higher and lower mass velocity types of once-through sliding pressure boilers. A design is needed so that special types of ribbed tubes are not required, thereby preventing an increase in the cost of the boiler. Also, a design is needed so that orificing or circulation pumps are not required.